When I was in college, I often began my homework at midnight. Nothing seemed to focus my thoughts on a term paper better than a morning deadline. I knew this routine wasn’t a wise one — after all, I might crash facedown in my textbook. But I told myself that adrenaline improved my writing. Besides, I figured I was losing weight on those nights with only four hours of sleep. I assumed that all that effort to stay awake and functional had to be zapping away the day’s calories.
Turns out that I was greatly mistaken. In fact, recent research shows that lack of sleep can make people gain weight, not lose it. Perhaps night-owl behavior like mine helps explain the famous “freshman 15” — the average number of pounds students gain in their freshman year of college.
People have acknowledged the value of sleep for centuries. But they’ve focused primarily on sleep’s impact on brain function and the obvious costs of burning the candle at both ends: lowered mental acuity, irritability, and a greater chance of accidents and mistakes. “If you talk to some neuroscientists today, the prevailing view is still that sleep is only for the brain,” says Eve Van Cauter, PhD, professor of medicine at the University of Chicago and an expert on the ways sleep affects endocrine function.
Over the last few decades, sleep researchers across the country have been overturning that view. Their studies indicate that curtailing sleep and getting poor-quality sleep are implicated in many diseases that affect the entire body, including type 2 diabetes, hypertension, cardiovascular disease, cancer and impaired immune function.
One of the most startling observations has come from Van Cauter and her University of Chicago colleagues. Over the course of four studies, they showed that people who don’t sleep enough, night after night, unwittingly trigger a hormonal storm that causes their appetites to rise.
Other researchers followed up with studies looking at the long-term health of large populations and found the implications of Van Cauter’s work borne out in real life: People who sleep fewer hours tend to become overweight or even obese. Even a difference of one hour is significant. Columbia University researchers, for instance, found that people between the ages of 32 and 59 who slept only four hours were 73 percent more likely to become obese than those sleeping seven to nine hours. Even a difference of two hours was significant. Those who slept only six hours were 23 percent more likely to become obese than those sleeping seven hours.
Does this mean we can shed pounds by getting additional shuteye? Maybe, but research hasn’t yet proven this supposition — the studies looking at whether overweight people shed pounds when they sleep more are just getting under way. Still, it’s clear that insufficient sleep encourages weight gain and that getting adequate sleep helps prevent it.
For most of us, adequate sleep means seven to nine hours a night, and over recent decades, fewer of us have been reaching that goal. According to research by the National Sleep Foundation, the average duration of sleep for Americans fell from a high of nearly nine hours in 1960 to seven hours in 2002, and to just over six and a half hours in 2009. More recent surveys show that the number of people sleeping fewer than six hours per night has doubled over the last four decades to nearly a third of the population.
“People tend to sacrifice sleep,” says Clete Kushida, MD, PhD, a sleep expert at Stanford’s Center for Human Sleep Research and a recent past president of the American Academy of Sleep Medicine. “Even people who pay attention to nutrition and exercise sacrifice sleep. They think they can get by with less, perhaps because the medical problems from sleep disorders usually become apparent [more slowly] over the years.”
Bleary-Eyed and Craving Cookies
Studying sleep is big business in the United States. The American Academy of Sleep Medicine has 8,000 members, and there are some 2,000 accredited sleep centers scattered across the country. Many are exploring the biochemical processes that go awry after too many nights of insufficient sleep. Others are investigating the body’s response to poor-quality sleep — sleep disturbed by stress, anxiety, a snoring partner, loud neighbors, or conditions like restless leg syndrome and sleep apnea.
Van Cauter set out to study the connection between sleep loss and appetite after anecdotal reports from sleep studies indicated that subjects were overeating during extended stays in the laboratory. The common assumption was that they ate because they were bored, but she decided to test that assumption. In the first-ever study to make the connection between sleep and appetite, published in 2004 in the Annals of Internal Medicine, Van Cauter’s team brought 12 lean and healthy young men into the lab for two four-hour nights of sleep followed by two 10-hour nights. They found that when the subjects slept for only four hours, they showed dramatic changes in two hormones that regulate appetite.
Blood draws revealed an 18 percent decrease in leptin, a satiety hormone produced by the stomach that tells the brain when the body has had enough food. They also showed a 28 percent increase in ghrelin, a hunger-causing hormone produced by our fat cells indicating that our energy reserves are running low and need to be replenished.
Taken together, these two hormones boosted the young men’s hunger — even though the amount they ate and exercised was the same during their nights of ample sleep. The subjects reported a 24 percent increase in appetite after less sleep, with a special eagerness for chips, cakes and cookies, and breads and pasta.
“This study suggests that there could be long-term consequences with prolonged sleep deprivation — especially if you’re trying to control your food intake or stick to a healthy diet,” says Kristen Knutson, PhD, a University of Chicago assistant professor of medicine who’s been involved in many sleep studies. “They were craving junk food, not apples and carrot sticks.”
Sleep researchers have also noted other important biochemical changes that might influence weight gain in people who are chronically sleep deprived. In 1999, Van Cauter and her University of Chicago researchers published a study of young healthy subjects who endured six nights with only four hours of sleep followed by six nights with 12 hours of sleep. During the short sleep days, examinations showed that the subjects’ ability to metabolize glucose was impaired, meaning that their muscles and other tissues weren’t able to remove glucose from the blood effectively.
This sort of sleep-related metabolic disruption can prompt the body to bump up its production of insulin, a hormone produced by the pancreas that flows through the blood and binds to our cells, allowing them to absorb glucose energy. Without that action, glucose builds up in the blood and prompts the pancreas to secrete more and more insulin.
Over time, this can create the kind of insulin resistance that marks adult-onset diabetes. Excess insulin also prompts the body to store fat.
Researchers aren’t entirely sure why sleep loss leads to this prediabetic condition, but they have observed that their sleep-stressed subjects have increased activity in their sympathetic nervous system, the mechanism that activates the fight-or-flight response. (This activation of the sympathetic nervous system might also account for the preference for junk food among Van Cauter’s sleep-deprived research subjects: Stressed people often crave the quick energy such fare offers.)
When sympathetic nervous activity increases, parasympathetic activity — which helps control the function of many of our organs — tends to drop. “Parasympathetic activity has an impact on the pancreas, so if it’s reduced, it’s possible that insulin is not being properly regulated,” says Knutson.
During their sleep-deprived state, the subjects also showed an increase in the level of the stress hormone cortisol in the early evening — a sharp contrast to the normal tapering down of this hormone before bedtime. The secretion of growth hormone (GH), which affects growth and metabolism, was also altered: Instead of the normal single burst of this hormone after sleep onset, GH was released twice, before and after sleep.
“These alterations in cortisol and growth hormone could affect insulin sensitivity negatively,” explains Knutson. “And that’s a bad thing; we want to be insulin sensitive.”
Researchers know that sleep deprivation disrupts one of the most basic mechanisms in our body: our internal clock. And, studies show that messing with our internal clock may have serious implications for our weight. We evolved over millions of years shaped by the earth’s cycles of day and night, and light and darkness, and our body’s clock still ticks according to those basic cycles.
This clock — often called our circadian rhythm — isn’t just a metaphor. It has a precise location in the brain’s hypothalamus, in two pinhead-size clumps of neurons called the suprachiasmatic nuclei (SCN) that sit above our two optic nerves. The SCN monitors the light coming in through our eyes and, based on the amount and timing of light, regulates vital rhythmic functions throughout the body, including temperature, the release of hormones, and metabolism.
The brain clock ticks away largely unaffected by the rest of the body — in fact, researchers have removed that portion of the brain from animals and watched as the SCN continues to pulse rhythmically on its own for a while. But the SCN is not the only clock in the body. Almost every cell has a clock-like function that operates on a 24-hour cycle. The difference between the brain clock and all these others is that the latter can’t operate on their own. They depend on the brain clock to sustain their rhythm.
“We think the main clock is like an orchestra conductor that keeps all the other instruments in time,” says Ilia Karatsoreos, PhD, a postdoctoral fellow at Rockefeller University’s Laboratory of Neuroendocrinology whose experiments with mice suggest that disrupting their circadian rhythms prompts weight gain and impulsive behavior. “Once that conductor is disrupted, it loses its ability to keep these other players in sync with each other. The organs and tissues are then not working as well together as they should be.”
By remaining awake when our biological clock says we should sleep, we risk scrambling the alignment of the internal systems regulated by our SCN — with terrible implications for our weight, among other things.
“All the different organs that regulate metabolism have circadian rhythms,” says Phyllis Zee, MD, PhD, professor of neurology and director of the Sleep Disorders Center at Northwestern University. “And when they’re out of sync, it can expose one to changes in metabolism or to choosing inappropriate food or to eating too much.”
Some researchers think late nights fueled by bright lights and glowing computer and TV screens may trick our bodies into thinking we’re in a sort of perpetual summer — a high-activity time when our hunter-gatherer predecessors would have been loading up on readily available carbohydrates in preparation for a long, cold winter.
“Our ancestors’ sleep durations would have been shorter in the summer,” says James Gangwisch, lead author of the Columbia study. And our caloric needs would have been far greater, he explains — both to fuel long days of activity and to accumulate precious fat stores that would carry us through the cold season.
Our modern reality is entirely different, of course. “Now,” notes Gangwisch, “we can have year-round fat deposition, preparing for a winter that never comes. It comes, of course, but we’re still warm and can get all the food we want and can still have short sleep durations because we have year-round light exposure.”
There are plenty of reasons why we’ve grown so estranged from sleep — despite its obvious health implications. Chief among these is our tendency to work longer hours. “Instead of working 40 hours, people are often working 50 to 60 hours per week,” Knutson says. “You want to have a life outside work, so you pay with sleep time.”
But the body keeps a very exact accounting of the hours needed for sleep. If we build up a sleep “debt” of an hour or two per night, Monday through Friday, we’re generally not going to be able to make it up in one weekend. We carry that debt and the burden of sleepiness forward, often not even realizing how sleep impaired we are.
“Several studies have shown that after cumulative sleep deprivation, individuals are no longer able to recognize the degree of sleepiness under which they operate,” says Van Cauter. “They think they’re OK, but when their performance is tested, they fail miserably.”
What we need, say some experts, is a new characterization of sleep — one that doesn’t regard it as a time when we just turn ourselves off. We need a new appreciation of slumber as a part of the environmental metronome guiding important cyclical functions in our body — functions that affect our weight, our body chemistry, our neurology and our overall well-being.
Most of us assume the routines of a lean lifestyle — like healthy meals and exercise — are limited to our waking hours. But that point of view leaves out the crucial dark side of our 24-hour cycle, when sleep prepares our bodies and minds to function at their best on the following day. It ignores the fact that our bodies require adequate downtime to regulate systems that have a direct impact on whether we accumulate unwanted weight, or succeed in evading it — now and over the long haul.
Kristin Ohlson is a writer based in Cleveland, Ohio.
Kristin Ohlson is a writer based in Cleveland, Ohio.